1. Field of the Invention
This invention relates to a gallium nitride substrate and an epitaxial wafer using the gallium nitride substrate.
2. Description of the Related Art
As a method of manufacturing a good and large gallium nitride substrate, methods are known, such as a method configured to obtain a nitride substrate by Disclosure Elimination by the Epi-growth with Inverted-Pyramidal Pits (DEEP) method (for example, refer to JP-B-3864870) or Void-Assisted Separation (VAS) method (for example refer to JP-A-2004-269313), and a method configured to obtain a GaN freestanding substrate by thickly growing a single crystal of gallium nitride on a heterogeneous substrate by Hydride Vapor Phase Epitaxy (HVPE) method, and separating it from the heterogeneous substrate.
In the gallium nitride substrate manufactured by the above-mentioned methods, dislocation density is varied from the rear surface toward the front surface in the thickness direction, and lattice constant is varied in the thickness direction, thus the freestanding substrate has warpage. In addition, a film thickness difference of not less than several tens micron meters exists in the gallium nitride substrate due to the fact that the single crystal of gallium nitride is thickly grown. Consequently, polishing treatment for planarizing the front surface side and the rear surface side of the gallium nitride substrate is needed. After the polishing treatment, peripheral processing is applied to the freestanding substrate so as to be shaped in a circular form having a desired diameter. After that, cleaning is carried out so that mirror surface is obtained.
As a method of polishing the gallium nitride substrate, a method configured to polish a substrate by loose grains supplied on a surface plate is known (for example, refer to JP-A-2001-322899). In the above-mentioned method, the substrate is polished such that the grain diameter of loose grain is gradually lessened and polishing speed is gradually reduced.